1. Field of the Invention
This invention generally relates to a method and apparatus for monitoring the etching condition of a chemical etching process, and more particularly, to a contactless real-time in-situ method and apparatus for the same.
2. Discussion of the Related Art
Etching rates and etch end points must be carefully monitored and controlled in order to end etching processes at a desired time. In semiconductor processing, inadequate or excess etching time can result in undesirable film patterning. For instance, for semiconductor devices having film layers or features in the micron and sub-micron range, an inadequate etch or an excess etch would result in the insufficient removal or the excess removal of a desired layer. Insufficient removal of a desired layer can result in an undesired electrical open or electrical short when the desired layer to be removed is an insulator or a conductor, respectively. Additionally, if the etch is in excess, undercutting or punch through can occur resulting in poorly defined film patterning or total lift-off. Inadequate or excess etching further leads to undesirable reliability problems in the subsequently fabricated semiconductor device. As a semiconductor wafer is extremely expensive due to many processing steps involved in the making thereof, the need to critically control the etching end point in an etching process is highly desirable.
The processing of electronic materials by wet etching can involve immersion, spray, or submerged spray systems. The use of spray etching is particularly attractive since the impingement provided by the spray enhances mass transport at the workpiece surface, thereby increasing the etch rate. The controlled hydrodynamic conditions of the impingement also provide a high degree of uniformity over large surface areas. With through mask etching, the impingement affords an improvement in the anisotropy of the etching process. This has resulted in the use of spray chemical etching for a wide variety of applications in the electronics industry.
The high etch rates introduced by the impingement systems demand an accurate determination of the end point in order to end a given etching process at the desired time. Inadequate or excess etching can result in undercutting or badly defined patterning. Etch rates, etch times, and etch end points are difficult to consistently maintain or predict due to lot-to-lot differences in film thickness and composition, as well as etchant temperature, hydrodynamics and concentration variability.
It would thus be desirable to provide an etch monitor and end point detection system for chemical etching with impingement which is a real time, in-situ system. Such a system should not interfere with the impingement of the etchant on the workpiece being etched. In addition, such a system should be robust and versatile, providing reliable results despite changes in film characteristics and being independent of etch process fluctuations that normally occur during etch processing.
Until now, there have been no monitoring methods or apparatus for spray etch systems which satisfy the above mentioned requirements. The techniques that are most widely practiced to control etching processes are etch termination determined by visual end point observation and simple timing based on predicted etch rates and strictly controlled parameters which affect the etch rates. In the visual method, the end point is detected by monitoring the change in color of the surface being etched. This method is limited to certain samples that show a sharp change in color at the end point and can only be used for transparent etchants. Even under these conditions, the application of this method for submerged spray etching may be restricted by visual obstruction due to gas evolution. In addition, any observation area may be very restricted, if at all available. The results of end point determination from a limited area can be misleading when either the film or the etching process are significantly nonuniform. The visual method is further limited for spray methods due to an accumulation of etchant droplets on the sight window from spray backsplash and mist.
On the other hand, the simple timing method requires prior knowledge of the initial film thickness and etch rate. Etch rate is estimated at significant cost by etching monitor wafers with pre-etching and post-etching thickness measurements. The thickness measurements and etch time must be accurately determined and the etch conditions be precisely maintained to use the timing method. Etch conditions (e.g., temperature, reagent concentration) often cannot be kept constant, even at great expense. Implementation of a reliable, in-situ end point detection technique which provides a robust etching method that is independent of such variables is desirable.
As with the etch systems mentioned above, an etch end point must be accurately predicted and/or detected to terminate etching abruptly. Etch rates, etch times, and etch end points are difficult to consistently predict due to lot-to-lot variations in film thickness and constitution, as well as etchant temperature, flow, and concentration variability. That is, an etch rate is dependent upon a number of factors, which include, etchant concentration, etchant temperature, film thickness, and the film characteristics. Precise control of any of these factors can be very expensive to implement, for example, concentration control.
Some currently used etch rate end point determination techniques depend on indirect measurement and estimation techniques. Some etch monitoring techniques have relied on external measurements of film thickness followed by etch rate estimation and an extrapolated etch end point prediction. However, etch rates may vary due to batch-to-batch differences in the chemical and physical characteristics of the film or the etchant. These extrapolation methods are inadequate.
As an alternative to indirect measurements and estimation techniques, real-time in-situ monitoring is preferred. Some in-situ techniques monitor the etch rate of a reference thin film. This may require additional preparation of a monitor wafer containing the reference thin film or a suitable reference may be unavailable. Still other techniques require physical contact of electrical leads with the wafer being etched and electrical isolation of those leads and associated areas of the wafer from the etchant. This presents problems associated with contamination, contact reliability and reproducibility, and the physical constraints which affect ease of use in manufacturing or automation.
It would thus be desirable to provide a method and apparatus which provides non-contact, real-time, in-situ monitoring of an etching condition of a wafer being etched.